Abstract: In recent years, UHVDC transmission has developed rapidly in China, and converter transformer is one of the core equipment of UHVDC transmission system. During long-term operation, the agglomeration deposition of fiber solid impurities on the electrode surface is likely to occur, which affects the tolerance of insulating oil. In this paper, the agglomeration characteristics of impurities and their mechanism are explored. The research results show that changing the electrode spacing will affect the distribution of electric field and the force of impurities, and the impurities will gather under the long gap and affect the formation of the "fiber bridge". The higher the concentration of impurities is, the higher the concentration degree will be; and the fiber impurities will gather more easily than the metal impurities. The research in this paper is helpful to clarify the agglomeration and deposition mechanism of impurities in insulating oil on the electrode surface. The results can provide a certain theoretical basis for the rational design of converter transformer outlet device, prevent insulation accidents of power equipment, and improve the long-term operation reliability of power equipment.